Tragesser, in U.S. Pat. No. 3,557,876 and Nahm in U.S. patent application Ser. No. 691,905, disclose the addition of pozzolan to drilling fluids. These pozzolans, when included in drilling fluids, provide a filter cake to prevent fluid loss from the wellbore wherein the filter cake has certain desirable properties. The filter cakes deposited from pozzolan-containing drilling muds can be made to be compatible with cements, and the filter cake itself can be converted into a cementitious material that bonds well with both wellbore cement and the formation. This eliminates problems of contamination of cement by incompatible filter cakes, and eliminates the need to displace mud and filter cake prior to cementing. Pozzolan, according to Tragesser, includes fly ash, flue dust, certain boiler and furnace slags, burnt ground brick, by-products of certain industrial processes, pumicites or volcanic ashes, pumice or pumice stones, obsidian, scoria, tuffs, some of the andesites, diatomites, cherts, shale, clays and pure opal. Nahm discloses particular advantages for the use of blast furnace slag as the pozzolan.
An improvement to the methods of using pozzolan-containing drilling muds is to convert the pozzolan-containing drilling muds directly to cementing slurries. Converting drilling muds directly to cementing slurries eliminates the need to dispose of used muds, eliminates the need to add some materials to the cement that are already present in the muds, such as fluid loss additives, and further improves compatibility of the muds with cementing slurries. Mud-based cements such as these are disclosed in U.S. Pat. Nos. 3,168,139; 3,499,491; and 5,058,679. U.S. Pat. No. 5,058,679 discloses a conversion of drilling mud directly to a cement slurry by adding blast furnace slag; curing agents such as sodium hydroxide, potassium hydroxide, zinc carbonate, or sodium carbonate; and optionally set control additives. The use of blast furnace slag cements instead of Portland cements significantly lowers cement costs, and further improves compatibility with pozzolan-containing drilling muds.
Determination of the amount of blast furnace slag in the drilling mud and in the cement slurry becomes important when blast furnace slag-containing drilling mud is converted to blast furnace slag cement. The amount of blast furnace slag remaining in mud will vary depending upon many factors. The amount of blast furnace slag in the final cementing slurry must be controlled to accomplish an acceptable slurry rheology, set time, and set cement properties.
An acceptable method was not previously known to determine the amount of an amorphous material such as slag in a slurry such as drilling muds, completion fluids, workover fluids or cement slurries. Such a method will be beneficial in conversion of muds containing amorphous materials such as blast furnace slag into blast furnace slag cement slurries.
Relative weight fractions of crystalline materials can be readily obtained by well-known X-ray diffraction techniques. But presence of a significant portion of an amorphous material such as blast furnace slag prevents obtaining useful information by existing X-ray diffraction techniques. X-ray diffraction has therefore not been a useful method in determination of the amounts of amorphous material in slurries such as cement slurries, drilling muds, completion fluids or workover fluids.
Use of X-ray diffraction as a quantitative analysis method for crystalline solids is disclosed by, for example, F. H. Chung, "Quantitative Interpretation of X-ray Diffraction Patterns of Mixtures. I. Matrix Flushing Method for Quantitative Multicomponent Analysis," J. Appl. Crystallogr., 7, p. 519-25 (1974). This method is not useful when a significant amount of amorphous material is present.
It is therefore an object of the present invention to provide an analytical method capable of determining the amount of an amorphous material such as blast furnace slag in a sample such as cement slurry, drilling mud, completion fluid, and workover fluid.